Determination of Cosmic-Ray-Produced Potassium and Calcium in Iron Meteorites.

Abstract

A Long lived radio-nuclide 40K and stable nuclides of calcium produced by cosmic ray in iron meteorites were determined by mass spectrometry using surface ionization technique with an electron multiplier. These nuclides were extracted simultaneously in a systematic wet chemical separation scheme with other stable nuclides of titanium, vanadium and chromium and long lived nuclide 53Mn. For the determination of absolute contents of the products, the dilution method was apllied using spikes, 39K and 42Ca. The contamination level of natural potassium and calcium was reduced as low as possible; the lowest, we obtained, was 0.2ppm potassium and 0.4ppm calcium. In such sample, the enrichment of 42Ca, 43Ca, 44Ca and 46Ca due to cosmic-ray products were observed at 70, 200, 20 and 200% respectively. It was not always possible to reach such low contamination level. On the other hand, the most of 40K extracted from iron meteorites was found to be the cosmic-ray products even at a higher contamination level. For the isotope fractionation and the isotope discrimination problem, long lived 40K seems not to be affected seriously because of the very high ratio of cosmic-ray product to natural 40K and of being placed between two stable nuclides of potassium. When we got the lowest contamination, cosmic-ray-produced 41K should be detected at the about 5% enrichment in mass 41 peak. However, we have no mean to correct the isotope fractionation and discrimination of potassium and could not determine the 41K in this work. According to the spallation systematics and also our earlier works, fortunately, 40Ca and 48Ca are produced by negligible amounts by cosmic-ray irradiation. The isotope fractionation and discrimination in the mass spectrometry were corrected by normalizing 40Ca and 48Ca peaks in samples to their reference values which were selected from literatures and our data of reagent calcium. When sample size was very small, mass 42 and 43 peaks were usually disturbed by tenacious hydrocarbon peaks. In order to eliminate such disturbances, the cavity of ion source in mass spectrometer and the filament had to be baked before the measurement in each run. Examples of results are as follows[unit: 1013atom/g of meteorite]. 40K: Grant A-350, 0.526, Grant Q-260. 0.541, Grant E-240, 0.470, Grant I-110, 0.42, Grant I-55, 0.458, Aroos, 0.715, Treysa, 0.546. 43Ca: Grant A-350, 1.77, Grant Q-260, 1.6, Grant I-110, 1.6, Grant I-55, 1.42, Aroos, 2.29, Treysa, 1.55. A dependency on the depth in the meteorite Grant was found to be about 20%, smoothly decreased from the surface to the center for both 40K and calcium. Our samples were not enough in number to determine the preatomospheric size of the meteorite yet. A fine structure in a cumulative spallation yield curve was examined in the isotopic ratios of cosmic-ray-produced calcium analyzed in a highly enriched sample. The yields of 42Ca and 44Ca were 10±5% higher than an expected yield of 43Ca. When we borrow the data by H. Voshage, the yield of 41K was also found to be apparently lower. Experimental procedures, especially the chemical procedure and the mass spectrometry of very small amount of these elements are described in detail.